Irisin ameliorates endoplasmic reticulum stress and liver fibrosis through inhibiting PERK-mediated destabilization of HNRNPA1 in hepatic stellate cells

Liver fibrosis is a common consequence of chronic liver diseases involved with the activation of hepatic stellate cells (HSCs) and endoplasmic reticulum (ER) stress. Irisin is a small polypeptide hormone that shows beneficial effects on metabolic disorders. The current study aimed to investigate the biological function of irisin on hepatic fibrosis. A mouse model of carbon tetrachloride (CCl₄)-induced hepatic fibrosis was established. CCl₄-treated mice showed elevated serum levels of AST and ALT, increased collagen accumulation, induced ER stress, and upregulated expressions of pro-fibrotic proteins in the liver compared to the controls. The administration of irisin, however, ameliorated CCl₄-induced hepatic fibrosis in both cultured HSCs and mice. PKR-like ER kinase (PERK) is a key component of the ER stress-associated signaling pathway. We found that irisin treatment improved the stability of heterogeneous nuclear ribonucleoprotein A1 (HNRNPA1) via regulating the phosphorylation of PERK in mouse livers and isolated HSCs. Also, the knockdown of HNRNPA1 eliminated the hepatoprotective effects of irisin on hepatic fibrosis and ER stress. In summary, this study showed that irisin alleviated ER stress and hepatic fibrosis by inhibiting PERK-mediated HNRNPA1 destabilization, suggesting that irisin may represent a promising therapeutic strategy for patients with liver fibrosis.

Integrated Stress Responses to Bacterial Pathogenesis Patterns

Activation of an appropriate innate immune response to bacterial infection is critical to limit microbial spread and generate cytokines and chemokines to instruct appropriate adaptive immune responses. Recognition of bacteria or bacterial products by pattern recognition molecules is crucial to initiate this response. However, it is increasingly clear that the context in which this recognition occurs can dictate the quality of the response and determine the outcome of an infection. The cross talk established between host and pathogen results in profound alterations on cellular homeostasis triggering specific cellular stress responses. In particular, the highly conserved integrated stress response (ISR) has been shown to shape the host response to bacterial pathogens by sensing cellular insults resulting from infection and modulating transcription of key genes, translation of new proteins and cell autonomous antimicrobial mechanisms such as autophagy. Here, we review the growing body of evidence demonstrating a role for the ISR as an integral part of the innate immune response to bacterial pathogens.

Bidirectional regulation of NF-κB by reactive oxygen species: a role of unfolded protein response

Nuclear factor-κB (NF-κB) is a transcription factor that plays a crucial role in coordinating innate and adaptive immunity, inflammation, and apoptotic cell death. NF-κB is activated by various inflammatory stimuli including peptide factors and infectious microbes. It is also known as a redox-sensitive transcription factor activated by reactive oxygen species (ROS). Over the past decades, various investigators focused on the role of ROS in the activation of NF-κB by cytokines and lipopolysaccharides. However, recent studies also suggested that ROS have the potential to repress NF-κB activity. Currently, it is not well addressed how ROS regulate activity of NF-κB in a bidirectional fashion. In this paper, we summarize evidence for positive and negative regulation of NF-κB by ROS, possible redox-sensitive targets for NF-κB signaling, and mechanisms underlying biphasic and bidirectional influences of ROS on NF-κB, especially focusing on a role of ROS-mediated induction of endoplasmic reticulum stress.

The ER luminal binding protein (BiP) alleviates Cd(2+)-induced programmed cell death through endoplasmic reticulum stress-cell death signaling pathway in tobacco cells

Cadmium (Cd) is very toxic to plant cells and Cd(2+) stress induces programmed cell death (PCD) in Nicotiana tabacum L. cv. bright yellow-2 (BY-2) cells. In plants, PCD can be regulated through the endoplasmic reticulum (ER) stress-cell death signaling pathway. However, the mechanism of Cd(2+)-induced PCD remains unclear. In this study, we found that Cd(2+) treatment induced ER stress in tobacco BY-2 cells. The expression of two ER stress markers NtBLP4 and NtPDI and an unfolded protein response related transcription factor NtbZIP60 were upregulated with Cd(2+) stress. Meanwhile, the PCD triggered by prolonged Cd(2+) stress could be relieved by two ER chemical chaperones, 4-phenylbutyric acid and tauroursodeoxycholic acid. These results demonstrate that the ER stress-cell death signaling pathway participates in the mediation of Cd(2+)-induced PCD. Furthermore, the ER chaperone AtBiP2 protein alleviated Cd(2+)-induced ER stress and PCD in BY-2 cells based on the fact that heterologous expression of AtBiP2 in tobacco BY-2 cells reduced the expression of NtBLP4 and a PCD-related gene NtHsr203J under Cd(2+) stress conditions. In summary, these results suggest that the ER stress-cell death signaling pathway regulates Cd(2+)-induced PCD in tobacco BY-2 cells, and that the AtBiP2 protein act as a negative regulator in this process.

Role of the ubiquitin-proteasome system in brain ischemia: friend or foe?

The ubiquitin-proteasome system (UPS) is a catalytic machinery that targets numerous cellular proteins for degradation, thus being essential to control a wide range of basic cellular processes and cell survival. Degradation of intracellular proteins via the UPS is a tightly regulated process initiated by tagging a target protein with a specific ubiquitin chain. Neurons are particularly vulnerable to any change in protein composition, and therefore the UPS is a key regulator of neuronal physiology. Alterations in UPS activity may induce pathological responses, ultimately leading to neuronal cell death. Brain ischemia triggers a complex series of biochemical and molecular mechanisms, such as an inflammatory response, an exacerbated production of misfolded and oxidized proteins, due to oxidative stress, and the breakdown of cellular integrity mainly mediated by excitotoxic glutamatergic signaling. Brain ischemia also damages protein degradation pathways which, together with the overproduction of damaged proteins and consequent upregulation of ubiquitin-conjugated proteins, contribute to the accumulation of ubiquitin-containing proteinaceous deposits. Despite recent advances, the factors leading to deposition of such aggregates after cerebral ischemic injury remain poorly understood. This review discusses the current knowledge on the role of the UPS in brain function and the molecular mechanisms contributing to UPS dysfunction in brain ischemia with consequent accumulation of ubiquitin-containing proteins. Chemical inhibitors of the proteasome and small molecule inhibitors of deubiquitinating enzymes, which promote the degradation of proteins by the proteasome, were both shown to provide neuroprotection in brain ischemia, and this apparent contradiction is also discussed in this review.

Modulation of endoplasmic reticulum chaperone GRP78 by high glucose in hippocampus of streptozotocin-induced diabetic mice and C6 astrocytic cells

Diabetes mellitus is known to increase the risk of neurodegeneration, and both diseases are reported to be linked to dysfunction of endoplasmic reticulum (ER). Astrocytes are important in the defense mechanism of central nervous system (CNS), with great ability of tolerating accumulation of toxic substances and sensitivity in Ca(2+) homeostasis which are two key functions of ER. Here, we investigated the modulation of the glucose-regulated protein 78 (GRP78) in streptozotocin (STZ)-induced diabetic mice and C6 cells cultured in high glucose condition. Our results showed that more reactive astrocytes were presented in the hippocampus of STZ-induced diabetic mice. Simultaneously, decrease of GRP78 expression was found in the astrocytes of diabetic mice hippocampus. In in vitro study, C6 cells were treated with high glucose to investigate the role of high glucose in GRP78 modulation in astrocytic cells. GRP78 as well as other chaperones like GRP94, calreticulin and calnexin, transcription levels were down-regulated after high glucose treatment. Also C6 cells challenged with 48h high glucose were activated, as indicated by increased level of glial fibrillary acidic protein (GFAP). Activated C6 cells simultaneously exhibited significant decrease of GRP78 level and was followed by reduced phosphorylation of Akt. Moreover, unfolded protein response was induced as an early event, which was marked by the induction of CHOP with high glucose treatment, followed by the reduction of GRP78 after 48h. Finally, the upsurge of ROS production was found in high glucose treated C6 cells and chelation of ROS could partially restore the GRP78 expression. Taken together, these data provide evidences that high glucose induced astrocytic activation in both in vivo and in vitro diabetic models, in which modulation of GRP78 would be an important event in this activation.

Sorafenib enhances proteasome inhibitor-mediated cytotoxicity via inhibition of unfolded protein response and keratin phosphorylation

Hepatocellular carcinoma (HCC) is highly resistant to conventional systemic therapies and prognosis for advanced HCC patients remains poor. Recent studies of the molecular mechanisms responsible for tumor initiation and progression have identified several potential molecular targets in HCC. Sorafenib is a multi-kinase inhibitor shown to have survival benefits in advanced HCC. It acts by inhibiting the serine/threonine kinases and the receptor type tyrosine kinases. In preclinical experiments sorafenib had anti-proliferative activity in hepatoma cells and it reduced tumor angiogenesis and increased apoptosis. Here, we demonstrate for the first time that the cytotoxic mechanisms of sorafenib include its inhibitory effects on protein ubiquitination, unfolded protein response (UPR) and keratin phosphorylation in response to endoplasmic reticulum (ER) stress. Moreover, we show that combined treatment with sorafenib and proteasome inhibitors (PIs) synergistically induced a marked increase in cell death in hepatoma- and hepatocyte-derived cells. These observations may open the way to potentially interesting treatment combinations that may augment the effect of sorafenib, possibly including drugs that promote ER stress. Because sorafenib blocked the cellular defense mechanisms against hepatotoxic injury not only in hepatoma cells but also in hepatocyte-derived cells, we must be careful to avoid severe liver injury.

Unravelling the story of protein misfolding in diabetes mellitus

Both environmental and genetic factors contribute to the development of diabetes mellitus and although monogenic disorders are rare, they offer unique insights into the fundamental biology underlying the disease. Mutations of the insulin gene or genes involved in the response to protein misfolding cause early onset diabetes. These have revealed an important role for endoplasmic reticulum stress in β-cell survival. This form of cellular stress occurs when secretory proteins fail to fold efficiently. Of all the professional secretory cells we possess, β-cells are the most sensitive to endoplasmic reticulum stress because of the large fluctuations in protein synthesis they face daily. Studies of endoplasmic reticulum stress signaling therefore offer the potential to identify new drug targets to treat diabetes.